That makes me really angry!! I wouldn't say I drive like a prat like that guy, but when I have children in the car my driving does change. They should give him the snip!!!
Apparently it disables itself if you put your foot down to exceed the limit. I think its just meant to be a guide. However it will probably be forced to be enforced at some point. You are right though, the few people which break the law are ruining it for the others which makes these aids become mandatory.